I'd probably bill 1 day if I did the work in dribs and drabs over 3 days and the total number of hours I spent were closer to 1 day than 2. You know how you'd feel if you worked 3 hours a day for 3 days: not like a superhero.

I don't know, this has just never happened with me.

Thanks ;)

You've put more than 1 standard 8-hour work day of effort into the project. If you round up, you have two days.

Although it's too close to one day, that it might be a bit unethical to say it were two...

Then do that.